Skin testing can be a helpful technique epidemiologically and diagnostically, provided that its limitations and liabilities are kept in mind. (1) A positive skin test, at best, is only a reflection of exposure to the antigen at sometime in the past. A negative skin test does not rule out infection. (2) Delayed hypersensitivity can be altered by intercurrent infections such as viral diseases or vaccines, overwhelming infection producing anergy, intrinsic immunologic breakdown, or exogenous causes such as medications. (3) Most antigens utilized for skin testing are impure extracts and, therefore, lack specificity. (4) Correct interpretation and administration of skin tests requires considerable experience and expertise. (5) They require two visits for the patient. Seldom should delayed hypersensitivity reactions of cellular immunity as we know them today be used as diagnostic tests. Their major contributions should be as an epidemiologic tool.